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LPG pipeline design temperature

LPG pipeline design temperature

LPG pipeline design temperature

(OP)
Recently I've been looking at LPG and Ethane pressurised liquid ambient temperature pipelines.

The issue which doesn't seem to be well addressed in various pipeline codes, including some specifically designed for LPG is what design temperature is required for the instance of an ambient temperature, pressurised liquid leaking due to defect / 3rd party damage etc.

I've been taking the view that the auto refrigeration temperature of the liquid at atmospheric pressure as it exits the pipe ( generally -40 to -70C) needs to be considered and the material has to have fracture toughness able to cope with that. This seems to be the approach people are now using / looking at in terms of dense Phase CO2 pipelines which also exhibit similar issues. Hence the design temperature of the pipe material should be set at say -40C for LPG and Charpy values taken accordingly. The thinking is about brittle fracture of the pipe and the hole becoming bigger or due to axial stress from cooling of the liquid the pipe actually breaks.

What's the thinking / experience here of others?

Am I being too cautious or given that in the event of a pipeline leak, the whole pipeline will slowly cool down to - xxC?

This post from a few years ago https://www.eng-tips.com/viewthread.cfm?qid=181137 has at the bottom a real life example of an ethylene pipe which was drilled into and froze to -110F.

Thanks for your input. LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

If you do a controlled blowdown, pipe and valves at the blowdown site may reach down to around -40C.
It is a good idea to design blowdown points to low temperature, however LPG pipelines are designed as just run of the mill "normal pipelines". Typical of what I've seen is one from US-Mexico that I have attached.
And this one in India https://idoc.pub/documents/lpg-pipeline-vnd50d383j...

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

(OP)
Yeh that's kind of what I'm finding and I'm not comfortable with it.

The same thing seems to apply with dense Phase CO2. No one used to be that concerned as there weren't very many, but once they started looking to expand them for carbon capture, all of a sudden everyone started getting worried about the low temperatures close to a leak point and also potential for running ductile failure.

I think LPG might be the same, i.e. there are very few long distance LPG lines and hence few if any failures or failures where the original hole caused issues later on as the stuff got colder and colder.

I'll have a read through those reports - many thanks.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

Is there enough of a leak risk to warrant designing all for low temps? Small leaks won't freeze a lot of pipe. If they turn into large leaks/breaks, it doesn't matter what the steel is. You have to replace that pipe anyway.

CO2 is different because there is a lot of explosive decompression force in compressed CO2 and it could start one of those rips that keeps on propagating, hence the need for higher toughness and/ or crack arrestors in CO2 pipelines.

Oh that's what you meant by "ductile failure" LPG doesn't do that as far as I know.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

(OP)
I think so. Depending on the size of the leak the whole pipeline may well get very cold, not just the bit by the leak itself.

It's the escalation of a small hole into a bigger hole or rupture is what concerns me. Even if you set the DT at -20 or -30, that's not really a bad thing in todays materials and then at least you stand a good chance that the pipe won't get brittle at -40.

LPG is strange stuff though - It either gets very cold and doesn't vent that much because there isn't enough heat getting into the pipe to vapourise it or it maintains pressure and hisses out at 4-5 bar like a giant gas bottle feeding your barbeque, but for a very long time. A bit different to CO2 alright, but equally I don't think setting a min DT at say 5C is a good idea either.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

I was recently involved with a pipeline project that during decompression could reach temperature below the minimum design temperature upstream of the orifice. I also share you concerns in this subject, LI.

That said, I think the associated costs with using a steel grade suited to this low temperature may hinder attempts of using it.

Daniel
Rio de Janeiro - Brazil

RE: LPG pipeline design temperature

(OP)
Well -30 is well within the range of reasonable API 5L steels nowadays without major cost impact. Much below -40 you run into trouble, but depends on the length of pipe involved.

I would much rather design for a rare but possible event than assume it won't happen.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

I would most likely just go with doing Charpy tests at -20C on API 5L. That is the Indian spec's min design temp. I don't see anyone else doing anything more than that.

Its not strange, just needs some vaporization heat to keep it going. You run out of pressure until more gas boils off the liquid. The key being you have have low pressure when it is at its coldest. You can ice things up the same with a natural gas pipeline blowdown, except the pressure never drops. Is that not worse than LPG?

Earth is also good insulation. Where's all that heat coming from? Local pressure might drop quickly. A temperature profile of a leak might be interesting.

B31.4& 8 don't really cover <-20. Not prohibited, but technically not covered.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

(OP)
What makes it worse than gas is that the whole line goes cold and hence contracts, so it's axial force not hoop.

However you can get pressurised fluid at 10 bar or so with a leak and local cooling of the metal.

But yes charpys at -20 or -30 should be good enough to prove it won't fail in leakage situations. I'm just trying to find out what other people do and if I'm trying to solve a problem that doesn't exist ( I don't think so but nice to get input).

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

Axial tension and hoop tension are on the same side of zero in Mohr's circle, so max shear stress is less than what an equal compressive axial stress in a hot line would produce.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

LI, IIRC API 5L is good up to -29C.

And don't get me wrong, I fully agree that this should be in the design case. The people with the money may not agree with us though and unfortunately to me they usually win the overall discussion.

Daniel
Rio de Janeiro - Brazil

RE: LPG pipeline design temperature

Right. Plus you must prove beyond any doubt (carefully) that they are wrong, whereas they have
only to tell your boss once that you are wrong.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

Lower Design temp for LPG lines should be -40degC. Additional cost for impact tested fine grain silicon killed CS piping should be minimal.

RE: LPG pipeline design temperature

georgeverghese, it sounds like you know what you're talking about. Can you share a reference to that effect? I would really like to understand why -20 is being used as the minimum design temperature in the links I provided, instead of your -40 number.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

(OP)
I'd like to know as well!

I think the -20 number is just custom and practice and seen as "run of the mill". Also the -29 figure comes from ASME B 31.4 which is probably the best LPG spec in the ASME family for pipelines. 400.1.2 says the code wasn't developed for piping below -30C. However 31.4 can be used for lower temps, but you just need to get the material testing done at lower temperaturees.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: LPG pipeline design temperature

I think it is because the typical LPG mix is more like 60% propane, which should keep toward the -20F minimum, rather than the -40s and lower. Contrarily, pure propane gets pretty cold.

I also remember this morning that C2/C3, LPG and NGL are often batched through common (liquid) carrier pipelines too, i.e design temp -20 and above.


Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

RE: LPG pipeline design temperature

Commercial LPG is mostly C3 - a depressure simulation should bear this lower design temp too. It would be alarming if someone selected -20degC. That design report for the LPG line in India appears to have no justification for the LDT of -20degC for this buried line.

RE: LPG pipeline design temperature

george, Thanks for your response.

I wouldn't say "no justification", only that "justification is not stated". Additionally noting that there is significant experience in India with long LPG pipelines, there is IMO no reason to suspect that they do not have justification of some kind. We just need to figure out what it is.

Reality used to affect the way we thought. Now we somehow believe that what we think affects reality.

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